Applicant's own prior U.S. Pat. No. 4,182,227, dated Jan. 8th, 1980, describes a ceiling mounted air-diffuser including an air-deflecting plate which is vertically adjustable below the ceiling level, so as to discharge air at an adjustable flow rate, horizontally along the ceiling. Also, means are provided to selectively vary the air flow rate, so as to obtain any desired directional flow pattern around the diffuser. One drawback of said known diffuser is the fact that its appearance varies in accordance with the adjusted vertical spacing of the air-deflecting plate from the celling, so that, when there are several air diffusers adjusted at various flow rates located in the same room, the room ceiling might be found unsightly.
U.S. Pat. No. 3,854,386 dated Dec. 17, 1974, entiled: AIR DIFFUSERS and issued to Allied Thermal Corporation, describes a ceiling mounted air diffuser in which the lower air-deflecting plate is set at a constant vertical spacing below the ceiling and in which air-flow rate adjustment is obtained by the provision of a large quantity of vertically-pivotable vanes, which makes the system difficult and time-consuming to adjust and in which the vanes are liable to produce a whistling sound when in nearly-closed position for minimum air-flow rate adjustment.